1. Field of the Invention
The present invention relates to improved design of a three dimensional piezoelectric positioning device for controlling the position of a tip member in a scanning tunneling microscope relative to the surface of a sample under observation Specifically, the present invention relates to the overall shape of the piezoelectric device, the electrode arrangement for the piezoelectric device and the mounting of the piezoelectric device.
2. Description of the Prior Art
In general, a scanning tunneling microscope operates by positioning a conducting tip or needle very close to the surface of a conducting sample. In particular, the tip may be held approximately ten angstroms (10 .ANG.) above the surface of the sample and at such a distance, if bias voltage is applied between the sample and the tip, a current referred to as the tunneling current will flow between the sample and the tip. Reference is made to the following article by Hansma and Tersoff, Journal of Applied Physics, Jan. 15, 1987 for a recent article giving a current review of scanning tunneling microscopes.
In general, scanning tunneling microscopes include positioning apparatus so as to control the scan of the tip in an XY raster scan over the surface of the sample, while keeping the tip a substantially constant height above the surface. In addition, a feedback loop is established to keep the tunneling current flowing between the tip and the surface constant. The tunneling current is very sensitive to the separation distance and may increase about a factor of five for a five angstrom (5 .ANG.) decrease in the separation.
This rapid change in tunneling current for changes in height provides a very sensitive indicator of the height for use in the feedback loop. Specifically, the feedback loop controls the height of the tip above the surface which control is referred to as the Z axis control. The signal which controls the vertical position or Z axis of the tip may then be used to determine the topography of the sample since the control signal is representative of the height of the surface as a function of the X and Y horizontal coordinates. The topography is, therefore, a function of all three coordinates X, Y and Z and it is important that the positioning device be able to control the tip in three independent orthogonal axis. In this way, the horizontal position and the vertical height can be determined by the control signals applied to the positioning device.
It is also to be appreciated that it is also possible to keep the tip stationary and move the sample with the positioning device in order to perform the scan. The feedback electronics would then keep the distance between the tip and sample constant by moving the sample vertically.
One typical design of a three dimensional positioning device is shown in the article by Binnig and Smith published in the Review of Scientific Instruments, August 1986. This article discloses the use of a tube of piezoelectric material with four electrodes o the outside of the tube and one electrode on the inside of the tube. The X and Y scanning of the tip is provided by applying the proper voltages to two of the outside electrodes (referred to as the AC electrodes). X and Y offsets of the tip are provided by grounding or applying high DC voltage to the other two outside electrodes (referred to as the DC electrodes). The vertical Z position of the tip is controlled by applying voltage to the inner electrode.
As shown in the Binnig et al article, the tip is connected to the edge of the tube to keep the mass of the tip mounting as low as possible. It is generally desirable to keep the mass on the end of the tube low so that the resonant frequency of the scanner is high to make the scanner more resistant to vibrations and to allow the scanner to operate at high speed. By applying voltages to the individual outer electrodes to create electric fields between the X and Y electrodes and the inner electrode, the piezoelectric material can expand or contract non-uniformly and cause the tube to bend to move the tip in the horizontal plane. By apply a voltage to the inner electrode the tube expands or contracts uniformly to lower or raise the tip.
A positioning device as described in the Binnig et al article, has a number of problems. In particular, the XY scan directions are not perpendicular and thereby create a raster scan which does not have orthogonal axis. This is because the sensitivity of the outer electrodes for moving the tip varies depending upon the electrode position around the cylinder with respect to the position of the tip. In addition, the positioning device shown in the Binnig et al article has large vertical-horizontal cross coupling.
For example, the vertical motion of the tip varies when the voltage on particular ones of the electrodes varies so that when these particular ones of the electrodes are energized to theoretically provide scanning in the horizontal direction, the height of the tip also varies. Conversely, when the Z electrode voltage is varied, the tube expands or contracts moving the tip not only in the vertical direction, but also in the horizontal direction. It can be seen, therefore, that the Binnig et al prior art positioning device is objectionable since it does not provide a structure with constant sensitivity and having the three scan directions orthogonal and independent and with no cross coupling.
Another similar design for a three dimensional scanning device is shown in the article by Besocke published in Surface Science 181 (1987). This scan device is also a tube with four outer electrode and one inner electrode but has the tip mounted in a central position. This, unfortunately greatly increases the tip mass and prevents easy access to the tip but does allow for the elimination of some of the problems with the Binnig et al type of device.